St Anne Street, Boundary Stone?

[michaelmace]

I found this on the bbc website.
A pic of a blue stone/plaque dated 1820. Apparently in Martindales car park in St Anne St.

Any ideas what it is??

I recall, long ago, a photo in the Maritime Museum with a similar plaque that mentioned the sinking of the ship 'Liverpool' off the coast of Africa in 1820. Perhaps the plaque is a relic from St Anne's Church where services to commemorate the occasion may have been held?